Evaluate Darwin configurations in CI without building

[RestartDK]

Summary

• replace both full nix-darwin system builds with lightweight configuration attribute evaluation
• match Edmund Miller's Darwin CI approach
• retain traitor check before the explicit host evaluation checks

Validation

• ./bin/traitor check --print-build-logs
• nix eval .#darwinConfigurations.dkumlin-macbook-pro.system --apply 'x: "OK"'
• nix eval .#darwinConfigurations.dkumlin-twin-macbook-pro.system --apply 'x: "OK"'
• git diff --check

Notes

• no Darwin system closure is built by these host checks
• no host rebuild was performed